The present invention relates to an improved heater for use with heated pressure transducers. More particularly, the present invention relates to an improved method for converting a pressure transducer configured for operating at a first temperature to a pressure transducer configured for operating at a second temperature, or for configuring a pressure transducer to operate at one or more temperatures.
FIG. 1 shows a sectional view of a prior art heated capacitive pressure transducer 100. Transducer 100 includes several major components such as an external shell 110, a heater shell 120, a heater 130, a capacitive pressure sensor 140, a front end electronics assembly 160, a heater control electronics assembly 170, and an input/output (I/O) electronics assembly 180. As will be discussed in greater detail below, transducer 100 generates an output signal indicative of a pressure measured by sensor 140.
For convenience of illustration, many mechanical details of transducer 100, such as the construction of sensor 140 and the mounting of sensor 140 and electronics assemblies 160, 170, 180, have been omitted from FIG. 1. However, heated capacitive pressure transducers such as transducer 100 are well known and are described for example in U.S. Pat. No. 5,625,152 (Pandorf); U.S. Pat. No. 5,911,162 (Denner); and U.S. Pat. No. 6,029,525 (Grudzien).
Briefly, external shell 110 includes a lower enclosure 112, an upper electronics enclosure 114, and a joiner 116 that holds enclosures 112, 114 together. Heater shell 120 is disposed within the lower enclosure 112 and includes a lower enclosure or can 122 and a cover 124. Heater 130 includes a barrel heater 132 and an end heater 134. Barrel heater 132 is wrapped around the external cylindrical sidewall of can 122 and end heater 134 is disposed on the bottom of can 122. Barrel heater 132 and end heater 134 are electrically connected via wires 136 so the two heaters 132, 134 may be simultaneously controlled via a single electrical signal. Sensor 140 and front end electronics assembly 160 are disposed within heater shell 120. Heater control electronics assembly 170 and I/O electronics assembly 180 are disposed within the upper electronics enclosure 114. A temperature sensor (e.g., a thermistor) 190 is fixed to an internal surface of heater shell 120.
Sensor 140 includes a metallic, flexible, diaphragm 142 and a pressure tube 144. Tube 144 extends from an area proximal to the diaphragm through the heater shell 120, and through the lower sensor enclosure 112. The lower, or external, end of tube 144 is generally coupled to a source of fluid (not shown). Pressure of fluid in the source is communicated via tube 144 to the lower surface of diaphragm 142 and the diaphragm 142 flexes up or down in response to changes in pressure within tube 144. Diaphragm 142 and a reference conductive plate of sensor 140 form a capacitor, and the capacitance of that capacitor varies in accordance with movement or flexion of the diaphragm. Accordingly, that capacitance is indicative of the pressure within tube 144. Front end electronics assembly 160 and I/O electronics assembly 180 cooperatively generate an output signal representative of the capacitance of sensor 140 which is, of course, also representative of the pressure within tube 144. I/O electronics assembly 180 makes that output signal available to the environment external to transducer 100 via an electronic connector 182.
FIG. 2 shows one example of how a capacitive pressure sensor 140 can be constructed. Capacitive pressure sensors of the type shown in FIG. 2 are discussed in greater detail in U.S. Pat. No. 6,029,525 (Grudzien). The sensor 140 shown in FIG. 2 includes a circular, conductive, metallic, flexible diaphragm 142, a pressure tube 144, and an electrode 246. Electrode 246 and diaphragm 142 are mounted within a housing 248. Electrode 246 includes a ceramic block 250 and a conductive plate 252. The ceramic block 250 is rigidly mounted to the housing 248 so that a bottom face of block 250 is generally parallel to, and spaced apart from, the diaphragm. The bottom face of block 250 is normally planar and circular. The conductive plate 252 is deposited onto the bottom face of block 250 and is also generally parallel to, and spaced apart from, the diaphragm. Conductive plate 252 and diaphragm 142 form two plates of a variable capacitor 254. The capacitance of capacitor 254 is determined in part by the gap, or spacing between, the diaphragm 142 and the conductive plate 252. Since the diaphragm flexes up and down (thereby changing the spacing between diaphragm 142 and conductive plate 252) in response to pressure changes in tube 144, the capacitance of capacitor 254 is indicative of the pressure within tube 144.
FIG. 2 shows only one of the many known ways of configuring a capacitive pressure sensor 140. However, capacitive pressure sensors 140 generally include one or more conductors that are held in spaced relation to a flexible, conductive, diaphragm. The diaphragm and the conductors form plates of one or more variable capacitors and the capacitance of those capacitors varies according to a function of the pressure in tube 144.
Returning to FIG. 1, ideally, the output signal of transducer 100 varies only according to changes in the pressure of the fluid in tube 144. However, changes in the temperature of transducer 100, or temperature gradients within transducer 100, can affect the output signal. This is primarily due to the different coefficients of thermal expansion of different materials used to construct the sensor 140. A secondary effect relates to the temperature sensitive performance of front end electronics 160. Accordingly, the accuracy of transducer 100 can be adversely affected by temperature changes in the ambient environment.
To minimize the adverse effect of changing ambient temperature, the temperature sensitive components of transducer 100 (i.e., sensor 140 and front end electronics 160) are disposed within heater shell 120, and in operation the heater 130 heats the heater shell 120 to a controlled, constant temperature. Heater 130 and heater shell 120 essentially form a temperature controlled oven that maintains the temperature of the temperature sensitive components at a constant preselected value.
Heater 130 is normally formed by placing wires or traces (e.g., copper) characterized by a selected electrical resistance onto a flexible, electrically insulating, thermally conductive shell. The traces are selected so that they will heat the heater shell 120 to a preselected temperature when a particular electrical signal is applied to the traces. The electrically insulating, thermally conductive shell is commonly made from thin layers of silicone rubber or Kapton (i.e., a polyimide high temperature film sold by Dupont under the trade name Kapton).
FIG. 1A shows a front view of a prior art barrel heater 132. As shown, the heater includes a heating element 132A that has been placed on an electrically insulating, thermally conductive shell 132B. Heater element 132A includes a resistive wire 132C disposed between two terminals 132D, 132E. The amount of heat produced by heating element 132A is primarily determined by the electric resistance provided between terminals 132D, 132E and the electric signal applied to the terminals 132D, 132E. End heater 134 is constructed in a similar fashion as barrel heater 132.
Heater 130 is normally permanently bonded to the external surface of beater shell 120 as indicated in FIG. 1. Heaters made using Kapton shells are normally bonded to the external surface of shell 120 with pressure sensitive adhesive. Heaters made using rubber shells are normally bonded to the external surface of shell 120 using a high temperature xe2x80x9cVulcanizationxe2x80x9d process that bonds the layer of rubber directly to the surface of shell 120. Rubber shell heaters are preferred for many applications because the adhesives used to bond Kapton heaters to the shell 120 tend to dry out, and thereby loose their bonding qualities, when exposed to high temperatures (e.g., one hundred fifty degrees Celsius).
In operation, heater control electronics assembly 170 applies an electrical signal to heater 130 via wires 172. Heater control electronics assembly 170 normally includes components for monitoring the temperature of heater shell 120 via sensor 190 and adjusting the signal applied to heater 130 so as to maintain the shell 120 at a constant temperature.
One popular form of heater control electronics assembly 170 includes (1) a plus/minus fifteen volt direct current power supply and (2) components for generating from the direct current supply an alternating current signal that swings between plus and minus fifteen volts. Assembly 170 normally applies via wires 172 (1) a minus fifteen volt reference signal to a first terminal of the heater 130 and (2) the alternating current signal to a second terminal of the heater 130. When the signal applied to the second terminal is equal to plus fifteen volts, maximum power is being applied to the heater and when that signal is equal to minus fifteen volts, no power is being applied to the heater. Assembly 170 may also include pulse width modulation controller circuitry for controlling the pulse width of the alternating current signal applied to the second terminal of heater 130. By modifying the pulse width and thereby adjusting the duty cycle of the pulse width modulated signal, this controller circuitry modifies the electrical power applied to the heater. This controller circuitry modifies the duty cycle in accordance with the temperature of shell 120 as measured by sensor 190 so as to maintain the temperature of shell 120 at a constant value. For example, when the heater is initially activated and the shell is at an ambient temperature, the signal applied to the second terminal may be a constant value of plus fifteen volts (i.e., a one hundred percent duty cycle signal). Such a signal applies maximum power to the heater so as to heat the shell to the desired operating temperature as fast as possible. As another example, once the heater shell has reached the desired operating temperature, assembly 170 may reduce the duty cycle of the signal applied to the second terminal to fifty percent (e.g., so that signal equals plus fifteen volts half the time and equals minus fifteen volts the rest of the time). In general, assembly 170 adjusts the duty cycle of the signal applied to the second terminal of the heater as needed to maintain the heater shell at the desired operating temperature. For convenience of exposition, application of power to the heater as discussed above shall be discussed herein in terms of applying an alternating current pulse width modulated signal to the heater.
One problem with prior art heated pressure transducers is that there is no easy way to convert a T1 transducer (i.e., a transducer configured for operation at a temperature T1, for example one hundred degrees) into a T2 transducer (i.e., a transducer configured for operation at a temperature T2, for example forty-five degrees). Once a particular heater 130 is bonded to the shell 120 and the transducer 100 has been assembled, the transducer 100 is effectively permanently associated with a particular temperature. For example, if a one hundred degree Celsius heater 130 (e.g., a heater that will generate a temperature of one hundred degrees Celsius when a pulse width modulated signal that swings between plus and minus fifteen volts is applied to the heater) is bonded to the shell 120 of a transducer 100, that transducer 100 is normally called a xe2x80x9cone hundred degree Celsius pressure transducerxe2x80x9d.
Since the heater 130 is permanently bonded to the shell 120, converting a T1 transducer to a T2 transducer generally involves (1) disassembling the transducer 100; (2) discarding the original heater shell and replacing it with a new heater shell to which a T2 heater has been permanently affixed; and (3) reassembling the transducer 100. This process is undesirably complex.
Another method of converting a T1 transducer to a T2 transducer is to use the pulse width modulation controller in electronics assembly 170 to adjust the electric signal applied to the heater 130 so as to heat the heater shell 130 to T2 instead of T1. Although such control is possible, it is generally too imprecise to be practical. For example, a heater that generates a temperature of one hundred degrees Celsius when a plus/minus fifteen volt, pulse width modulated signal is applied to the heater is capable of generating an average temperature of forty-five degrees (or any other average temperature value that is greater than ambient and less than one hundred degrees) Celsius if the duty cycle of the pulse width modulated voltage signal applied to the heater is appropriately adjusted. However, although adjusting the duty cycle of the pulse width modulated voltage signal can cause the heater to generate an average temperature of forty-five degrees, the instantaneous temperature of the heater will normally oscillate above and below forty-five degrees by as much as several degrees. So, although the pulse width modulation controller in electronics assembly 170 provides a sufficient degree of control to maintain a T1 heater at temperature T1 while the ambient temperature is changing, the amount of control provided by that controller is generally not sufficient to permit using a T1 heater to generate a constant temperature of T2. Heated pressure transducers operate most accurately when their temperature sensitive components are heated to a constant temperature, rather than to a temperature that oscillates by relative large amounts around an average value. Ideally, the temperature of the heater shell oscillates around the desired temperature value by no more than one tenth of a degree Celsius. Accordingly, using such electronic control to convert a T1 transducer to a T2 transducer is generally not practical.
Another method of converting a T1 transducer to a T2 transducer is to include additional components in heater control electronics assembly 170 (e.g., additional power supplies or controllers that are more sophisticated than the pulse width modulation controller discussed above) and to use those components to appropriately adjust the electric signal applied to the heater. However, it is generally preferred to use only very simple, compact circuitry in transducer 100, and such additional components would require more space, more circuitry, consume more power, and generate more heat than is desirable.
Accordingly, it would be advantageous to provide less expensive and more accurate methods and structures for converting a T1 pressure transducer to a T2 pressure transducer.
These and other objects are provided by an improved heater for use with pressure transducers. The heater includes a first heating element and a second heating element. The first heating element provides a first electrical resistance and the second heating element provides a second electrical resistance. In preferred embodiments, the first resistance is different than the second resistance.
If each of the two heating elements provides a different electrical resistance, each of the two elements will be associated with a different, unique, operating temperature. This allows the amount of heat that is produced by the heater to be selectively changed simply by changing the electrical connections between the heater and the electronic circuitry of the pressure transducer. More specifically, the heater can be electrically connected to the pressure transducer in at least four different configurations, and each configuration is associated with a unique operating temperature. The pressure transducer can be heated to a temperature of (a) T1 by connecting the first heating element in series with the heater control electronics assembly so that the heater provides an electrical resistance of R1; (b) T2 by connecting the second heating element in series with the heater control electronics assembly so that the heater provides an electrical resistance of R2; (c) T3 by connecting the first and second heating elements in series with the heater control electronics assembly so that the heater provides an electrical resistance of R3; and (d) T4 by connecting the first and second heating elements in parallel with the heater control electronics assembly so that the heater provides an electrical resistance of R4.
Each of the multiple operating temperatures provided by the invention is achieved by using the heater to provide a different, unique, electrical resistance. This stands in contrast to prior art methods of providing multiple operating temperatures like using a heater that provides a single electrical resistance and electronic controllers for varying the signal applied to the heaters. Whereas prior art methods produce a temperature that oscillates around the desired operating temperature by as much as several degrees Celsius, a heater constructed according to the invention may be used to provide a transducer that can selectively operate at any one of multiple operating temperatures without deviating from any of the selected operating temperatures by more than a tenth of a degree Celsius.
Some benefits of the invention may be obtained simply by using a new heater constructed according to the invention with prior art pressure transducers. In other words, other than changing the heater, prior art pressure transducers need not be modified in any way to obtain benefits of the invention. For example, the same electronic circuitry used to control the heater of a prior art heated pressure transducer may be used to control heaters constructed according to the invention.
However, additional benefits of the invention can be obtained by making minor modifications to the electronic circuitry of a prior art heated pressure transducer. For example, in addition to providing a pressure transducer capable of operating at multiple temperatures, the invention may be used to decrease the xe2x80x9cwarm upxe2x80x9d time (i.e., the time required to heat the heater shell from the temperature of the ambient environment to the operating temperature) of a pressure transducer. For example, if the first heating element is associated with the temperature T1 and the second heating element is associated with the temperature T2, where T1 is higher than T2, the warm up time for a T2 transducer may be decreased in accordance with the invention by using the first heating element to heat the heater shell from the temperature of the ambient environment to the temperature T2 and thereafter using the second heating element to maintain the temperature of the heater shell at T2. Since T1 is higher than T2, the first heating element (the T1 heater) normally generates more heat than the second heating element (the T2 heater), and using the T1 heater in this fashion decreases the transducer""s warm up time (since it applies more heat to the heater shell during the warm up time). To use this method of decreasing the warm up time, the electronic circuitry in the transducer preferably includes components for selectively switching between electrically connecting the first heating element to the heater control electronics and electrically connecting the second heating element to the heater control electronics. Also, the electronic circuitry of the transducer is preferably capable of switching between the first and second heating element based on the temperature of the heater shell as measured by a temperature sensor.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein several embodiments are shown and described, simply by way of illustration of the best mode of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.